In many industrial, military, security or medical applications, images of the internal structure of objects are required. Radiography is often used for imaging. Radiography generally comprises either conventional transmission radiography or backscatter radiography.
FIG. 1 is a schematic illustrating the configuration used for conventional transmission radiography. In conventional radiography, an image is formed by transmitting radiation from a radiation generator 105 through an internal detail 110 within object 130. Attenuated radiation is received by a radiation detector 115 which is disposed on the side of the object opposite to that of the radiation generator 105. In the case of tomography, the object 130 is generally rotated about axis perpendicular to the plane of the figure.
FIG. 2 is a schematic illustrating the configuration used for backscatter radiography. Unlike conventional radiography which relies on transmission, in backscatter radiography radiation is scattered by internal detail 210 within object 230. In backscatter radiography, the radiation generator 205 and radiation detector 215 are on the same side of the object 230. All backscatter radiography techniques allow one-sided imaging of the object since the radiation generator 205 and the radiation detector 215 are located on the same side of the object 230. This is the same imaging configuration that people and animals use for optical viewing of their surroundings. In backscatter radiography, illumination of an entire region of the object to be interrogated in a single exposure has generally only been possible using a pinhole, coded aperture, or a restriction positioned between the object and the radiation detector. This generally results in either extremely inefficient sensing of the radiation or the introduction of substantial image-obscuring structured noise, thus requiring large exposure times for typical radiation sources. An alternative includes use of a scanning pencil or fan beam for illuminating a temporal sequence of points or lines on the object surface. This also yields long exposure times and decoding algorithms having long calculation times, besides requiring an expensive scanning apparatus.
U.S. Pat. No. 6,735,279 to Jacobs et al. discloses a snapshot backscatter radiography (SBR) system and related method that includes at least one penetrating radiation source, and at least one substantially transmissive radiation detector. The substantially transmissive radiation detector is interposed between an object region to be interrogated and the radiation source. The substantially transmissive radiation detector receives and detects illumination radiation from the radiation source before transmitting a portion thereof to interrogate the object region, wherein a portion of backscattered radiation provided by the object region is detected by the detector. An image of the object can be obtained by subtracting the illumination radiation detected at the detector, or an estimate thereof, from a total of all radiation detected by the detector. U.S. Pat. No. 7,130,374 to Jacobs, et al. is based on the SBR system disclosed in U.S. Pat. No. 6,735,279 to Jacobs et al., but further discloses a changeable collimating grid having a plurality of spaced apart radiation absorbing features coupled to a structure changing a position of the plurality of features disposed in at least one of the paths of the illumination radiation and the path of the backscattered radiation.
FIG. 3 is a schematic illustrating a SBR system as disclosed in U.S. Pat. No. 6,735,279 to Jacobs et al. A snapshot backscatter radiography (SBR) system 300 includes at least one penetrating radiation source 310 and at least one substantially transmissive radiation detector 320, such as a flexible detector sheet. Both the radiation source 310 and the radiation detector 320 are disposed on the same side of the object 330, which includes internal detail 332. The surface of the object or the surface of the medium covering the object, such as the earth, is indicated as reference 335.
System 300 can include a radiation source controller 350 and computer 360. Computer 360 preferably includes memory and provides various system functions, such as producing data representing an image of the object interrogated based on radiation data detected by detector 320. A display screen 380 for representing an image of the object interrogated is also preferably provided.
The substantially transmissive radiation detector 320 is preferably a digitizing radiation detector-film screen. In this embodiment, computer 360 has at least modest speed and data storage capacity for data processing and driving a high resolution display 380.
Two-dimensional (2D) or three-dimensional (3D) data sufficient to generate an image of the internal structure of objects capable of scattering a portion of incident radiation is acquired in a single exposure illumination of an interrogated area of the object surface. Image data for 2D back-projections can be acquired in a single radiation generator/source burst.
System 300 is generally contained in a protective and supportive housing 345. Housing 345 holds the various components of system 300 in place.
The spacing of detector 320 from source 310 generally depends on the area to be illuminated. For most wide area applications, the spacing from source 310 to detector 320 is generally about the same order of magnitude as the length of detector 320.
The arrangement shown in FIG. 3 implies that the radiation/object interactions of primary consequence are scatterings, although absorption can also be significant. Radiation from the radiation source 310, shown by reference 315, is directed at detector 320 which detects illumination radiation 315. This detected radiation pattern is referred to as the illumination signal. The detected illumination signal includes information on the spatial variations of the illumination radiation field and the spatial variation of structure and sensitivity of the detector 320.
The substantially transmissive radiation detector 320 transmits a portion of the illumination radiation received, shown as reference 325, which penetrates surface 335 and strikes internal detail 332 of object 330. The detector medium is generally a detector sheet that provides an area that is at least equal to the illumination area provided by radiation source 310.
The internal detail 332 of object 330 then backscatters a portion of the transmitted radiation 325, shown as reference 355. Preferably, object 330 scatters (backscatters via single or multiple collisions) at least 5% to 30% of the illuminating field provided by radiation source 310. For example, a portion of radiation 325 is transmitted through object 330 and is identified in FIG. 3 as transmitted radiation 365. Substantially transmissive radiation detector 320 detects some of the backscattered radiation portion 355, the backscattered radiation pattern referred to herein as the backscatter signal. Thus, the backscatter signal is generated by the backscattered radiation field 355 that emerges from the object or other surface 335 after being scattered by the internal structure of object 330. The desired image of the object can be computed by subtracting the illumination signal, or an estimate thereof, from the total detector response measured. The total detector response measured comprises a superimposed sum of the illumination signal and the backscatter signal radiation, which includes information on the spatial variations of the radiation field and structure of the detector as well as information on the object structure. Alternatively, the desired image can be obtained by subtracting a suitably normalized incident (illumination) radiation signal, or estimate thereof, from the backscatter signal collected by the detector.
Although SBR provides advantages over conventional transmission or backscatter radiography, in SBR only a small fraction of the detection dynamic-range is employed in the algebraic formation of the backscatter image because the backscatter signal is superimposed on the illumination signal. As a result, significant image processing is required to form an image, and images can have significant feature obscuring noise.